1. Field of the Invention
The invention relates to implantable prostheses. In particular, the invention relates to endoluminal stent-grafts which are deployed in blood vessels to bridge aneurysms. The invention is particularly related to a method for using stent-grafts which incorporate a structure for restricting dilation of the stent-graft after it is implanted.
2. State of the Art
Transluminal prostheses are well known in the medical arts for implantation in blood vessels, biliary ducts, or other similar organs of the living body. These prostheses are commonly known as stents and are used to maintain, open, or dilate tubular structures or to support tubular structures that are being anastomosed. When biocompatible materials are used as a covering or lining for the stent, the prosthesis is called a stent-graft. If used specifically in blood vessels, the stent-graft is known as an endovascular graft. A stent or stent-graft may be introduced into the body by stretching it longitudinally or compressing it radially, until its diameter is reduced sufficiently so that it can be fed into a catheter. The stent-graft is delivered through the catheter to the site of deployment. If the stent-graft is a self-expanding stent-graft, when it is released from the catheter, it self-expands. If the stent-graft is not self-expanding, upon release from the catheter it is mechanically expanded (e.g., by a balloon). Regardless of whether they are self-expanding or not, stent-grafts introduced in this manner are known as endoluminal stent-grants.
A typical state of the art self-expanding stent, such as disclosed in U.S. Pat. No. 4,655,771 to Wallsten or in U.K. Patent Number 1,205,743 to Didcott, is shown herein in prior art FIGS. 1, 1a, 2, and 2a. Didcott and Wallsten disclose a tubular body stent 10 composed of wire elements, e.g., 12, 13, each of which extends in a helical configuration with the centerline 14 of the stent 10 as a common axis. Half of the elements, e.g. 12, are wound in one direction while the other half, e.g. 13, are wound in an opposite direction. With this configuration, the diameter of the stent is changeable by axial movement of the ends 9, 11 of the stent. Typically, the crossing elements form a braid-like configuration and are arranged so that the diameter of the stent 10 is normally expanded as shown in FIGS. 1 and 1a. The diameter may be contracted by pulling the ends 9, 11 of the stent 10 away from each other as shown by arrows 16, 18 in FIG. 2. When the ends of the body are released, the diameter of the stent 10 self-expands and draws the ends 9, 11 of the stent closer to each other. The contraction to stretching ratio and radial pressure of stents can usually be determined from basic braid equations. A thorough technical discussion of braid equations and the mechanical properties of stents is found in Jedweb, M. R. and Clerc, C. O., “A Study of the Geometrical and Mechanical Properties of a Self-Expanding Metallic Stent—Theory and Experiment,” Journal of Applied Biomaterials; Vol. 4, pp. 77-84 (1993). In general, however, the contraction to stretching ratio is related to the axially directed angle α between the crossing elements 12, 13 in the expanded state as shown in FIG. 1. As explained in Didcott, the greater the magnitude of the angle α, the greater the amount of axial extension will be required to contract the diameter of the stent.
Prior art FIG. 3 shows a state of the art stent-graft 20 which includes a braided mesh stent exoskeleton 22 and an inner biocompatible liner 24. The stent shown is a Didcott-type stent with a crossing angle β≈90°, one straight 26 and one flared end 28. The liner (graft material) is polyethylene terepthalate (PET), polycarbonate urethane, or expanded polytetrafluroethylene (EPTFE) material which is attached to the stent by means of sutures or adhesives. However, those skilled in the art will appreciate that the state of the art stent-grafts include stents which are made of various different materials and which include self-expanding as well as balloon expandable stents. In addition, the graft materials of the state of the art stent-grafts include polyurethane, silicone rubber, polypropylene, polyolefin, collagen, elastin, etc. The graft material may be non-woven, woven, spun, knitted, braided, expanded, etc. It can be attached to the inside of an “exoskeleton” stent or to the outside of an “endoskeleton” stent by sutures, adhesives, co-braids, staples, etc.; or it can be attached within (i.e., both inside and outside) the skeleton if desired.
As shown in FIGS. 4-7, the exemplary stent-graft 20 of FIG. 3 is deployed with the aid of a catheter 30 and a guide wire 32. As shown in FIG. 4, the guide wire 32 is maneuvered through blood vessels to a location in artery 34 beyond aneurysms 36, 38. The stent graft (not shown in FIG. 4) is carried inside the catheter 30 which is guided over the guide wire (with the aid of fluoroscopy) to the site of the aneurysms 36, 38. The stent-graft is deployed as shown in FIGS. 5-7, by releasing one end 28 of the stent-graft 20 on one side of the aneurysms 36, 38. This is usually accomplished with the aid of a pusher (not shown) which moves inside the catheter and pushes the stent-graft out of the catheter as the catheter is pulled back. As the stent-graft 20 is released from the catheter 30, it expands as shown in the Figures and bridges the aneurysms 36, 38. Over time, i.e. several months, the porous liner 24 of the stent-graft 20 clots with blood and tissue ingrowth occurs. The liner thereby becomes non-porous or microporous allowing nutrient passage but no fluid leakage through the stent-graft into the aneurysms. It has been observed that, in many cases, after the graft has “healed-in,” the stent-graft begins to dilate as shorten.
As illustrated in FIGS. 8-11, significant dilation of the stent-graft can result in a catastrophic failure. For example, FIG. 8 shows the stent-graft 20 installed in the abdominal aorta 40 bridging an aneurysm 42. During the first few months after implantation, the stent-graft 20 begins to heal-in. After six to twenty-four months, however, the stent-graft 20 may begin to dilate and shorten as shown in FIG. 9. This is particularly likely if the aneurysm 42 is empty rather than filled with organized clot. If unrestrained, the dilation of the stent-graft 20 will ultimately result in the end 26 of the stent-graft 20 slipping into the aneurysm 42 as shown in FIG. 10. This dislocation of the stent-graft can be catastrophic (even fatal) as it allows the aneurysm to be repressurized with blood, thereby risking rupture. While it is generally believed that a flared end of a stent-graft is more secure than a non-flared end, observations by the inventor suggest that this is not true. As shown in FIG. 11, the flared end 28 of the stent-graft 20 can become dislodged and slip into the aneurysm 42 with the same catastrophic results.
It is not entirely understood why stent-grafts dilate and become dislodged over time. It is believed that the dilation is the result of blood pressure acting on the wall of the stent-graft. At a low crossing angle of the stent wires, e.g., 90°, pressure inside the stent-graft will cause a dilation and shortening of the stent-graft. At a higher crossing angle of the stent wires, e.g., 120°, the pressure inside the stent-graft causes a lengthening of the stent-graft, which can result in a dislodging of the stent-graft. In this case, it is believed that the pulsing of blood pressure causes the stent-graft to alternately lengthen and shorten. This pulsing motion is believed to cause the stent-graft to “walk” along the arterial wall. Whatever mechanisms are involved, it is clear to the inventor that dilation of the installed stent-graft is a cause of stent-graft failure because of dislodgement.